rt2x00: Initialize txop during conf_tx() callback

The txop parameter is supported by rt61pci and rt73usb,
and thus should be written to the register instead
of using the fixed value set during initialization.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>